The present invention relates to data processing systems of the type having a plurality of units of different types communicating with each other over a common bus under the control of a priority controller.
The prior art discloses a data processing system wherein a bidirectional asynchronous common bus has memory, central processor and various I/O controllers connected to it. A distributed priority network grants access to the bus depending upon the highest priority unit making a request. A master or requesting unit may address a slave unit by generating its address and a function code. In addition, if the master requires a response from the slave, the master may supply its own address to the slave. Any bus cycles available between the request and response may be utilized by other units. The system employs a lock operation wherein a master unit may lock a slave unit for moe than one bus cycle. However, only one lock may be in effect at any one time even though other units may still communicate with each other on bus cycles not utilized in the "locked" operation. A lock operation is one where a link is established between two units and other units are prevented from communicating with these units until the lock is terminated.
The present invention extends the concept of locked operation in a manner which permits more than one pair of units to be locked at any given time while any unlocked unit may communicate with any other unlocked unit.